1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, arc fault circuit breakers.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. As such, they are subjected to heavy use and, therefore, must be capable of performing reliably over many operating cycles. They also must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
The circuit breaker push-pull handle is moved from in-to-out in order to open the load circuit. This action may be either manual or, else, automatic in the event of an overload or fault condition. If the push-pull handle is moved from out-to-in, then the load circuit is re-energized. If the load circuit had been automatically de-energized, then the out-to-in operation of the push-pull handle corresponds to a circuit breaker reset action.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection. Arc faults are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire.
Although many circuit breakers also employ ground fault protection, in aircraft applications, the aircraft frame is ground, and there is no neutral conductor. Some aircraft systems have also provided ground fault protection, but through the use of additional devices, namely current transformers which in some cases are remotely located from the protective relay.
During sporadic arcing fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection - ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. It is still desirable, however, to provide separate indications in order to distinguish an arc fault trip from an overcurrent-induced trip.
Finally, there is an interest in providing an instantaneous trip in response to very high overcurrents such as would be drawn by a short circuit.
The challenge is to provide alternative protection and separate indications in a very small package, which will operate reliably with heavy use over a prolonged period. A device which meets all the above criteria and can be automatically assembled is desirable.
In aircraft applications, two practical considerations make automatic operation difficult to achieve and, possibly, undesirable. First, the design of a conventional aircraft circuit breaker makes it difficult to add an externally initiated tripping circuit thereto. Second, certain circuits on an aircraft are so critical that manual intervention by a crewmember may be desirable before a circuit is de-energized.
U.S. Pat. No. 5,546,266 discloses a circuit interrupter including ground fault and arcing fault trip circuits, and indicators, such as LEDs, to produce an indication of the cause of the trip.
U.S. Pat. No. 5,831,500 discloses a circuit breaker employing a trip flag, a status insert and a status flag that are viewable through a lens based upon the trip, open, and closed positions, respectively, of the circuit breaker.
U.S. Pat. No. 5,847,913 discloses a circuit breaker employing ground fault interruption and arc fault detecting circuitry. Conduits are provided in the circuit breaker housing for conveying light or reflecting light between light sources, plungers or bimetal disks and an opening of the housing.
U.S. Pat. No. 6,084,756 discloses a tester for an arc fault circuit breaker in which an indicator is extinguished when a circuit breaker responds to an arc fault condition.
There is room for improvement in circuit breakers.
The present invention is directed to a circuit breaker, which employs a dual trip, dual latch trip magnetic actuator. The magnetic actuator releases both a first indicator latch, which controls an arc fault trip indicator, and a second primary latch, which holds the circuit breaker closed. When an arc fault trip condition is detected, the coil of the magnetic actuator is energized, and the first latch for the trip indicator is attracted to the core of the magnet. Moving the first latch, in this manner, releases an indicator to indicate that an arc fault has occurred. Also, the second primary latch is attracted to the core of the magnet, thereby opening the circuit breaker separable contacts.
According to the invention, a circuit breaker comprises: a housing; separable contacts mounted in the housing; a latchable operating mechanism including a latch member which when released opens the separable contacts; an overcurrent assembly responsive to selected conditions of current flowing through the separable contacts for releasing the latch member to trip the separable contacts open; an arc fault indicator; an arc fault trip actuator which when energized moves a first latch for releasing the arc fault indicator and a second latch for releasing the latch member to trip the separable contacts open; and an arc fault current assembly responsive to selected arc fault conditions of current flowing through the separable contacts for energizing the arc fault trip actuator to actuate the arc fault indicator and to trip the separable contacts open.